Re: Could not find the main class. Program will exit.

Knute Johnson <>
Sat, 30 Jan 2010 13:14:08 -0800
On 1/30/2010 12:05 PM, Lew wrote:

Knute Johnson wrote:

I'm curious on a Windows system how you use two JDKs or JREs on the
same computer?

Essentially the same way as on any other platform - you install them
into different directories.

markspace wrote:

First, Sun designs their installer and JRE/JDKs to co-exist
peacefully, so it doesn't happen on accident. I only use the
auto-update mechanism

Tell each installation a different directory for its Java.

It is probably best to install them in version order, save that you
would want to install your preferred version last.

from Sun, and my Program Files/Java directory looks like this:

Brenden@Homer /cygdrive/c/Program Files/Java
$ ls
jdk1.5.0_12 jdk1.6.0_17 jre1.6.0 jre1.6.0_03 jre1.6.0_07
jdk1.5.0_22 jre1.5.0_12 jre1.6.0_01 jre1.6.0_04 jre1.6.0_16
jdk1.6.0 jre1.5.0_22 jre1.6.0_02 jre1.6.0_05 jre6

(I'm running Cygwin, btw.)

However, the technique is a Windows technique, not a Cygwin technique.

In NetBeans, there's a little drop down that lets you choose your
target environment. I can switch between 1.5 and 1.6 on my copy of
NetBeans. This requires the runtimes be available, which is why I need

Eclipse has a JRE platforms option in its preferences dialog that serves
the same purpose.

In both IDEs, each project can target a different platform.

For deploying applications, you can specify Specification-Version in
the manifest to set the minimum compatible version of JRE required. I
think at least for applets, you can set the maximum version too,
allowing to to specify that only 1.5 would be used even if 1.6 is
available, for example.

For the latter reason, the OP may need both JREs on his system.

If you install them in the normal manner, only the last one is set up in
the path and registry. If you don't use an IDE how do you get at the
version you want? Do you just specify the path to the compiler or
runtime programs?



Knute Johnson
email s/nospam/knute2010/

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